The measurement of a distance or displacement is fundamental to various forms of transducers for measuring numerous mechanical variables such as force and weight, pressure or acceleration, because such variables can easily be converted to proportional displacements by calibrated springs, membranes or masses. This also applies to other variables such as thermal, electric and magnetic variables. In view of the trend toward rapidly increasing automation, the measurement of displacement thus becomes a basic problem in measuring techniques.
Important considerations in this connection are reliability and the freedom from interference of both the measurement itself and also of the transmission of the measured values to an indicating unit or evaluation computer. This problem exists particularly in power supply and industrial process sytems which often provide an electromagnetically disturbed environment, such as in the vicinity of pulse-controlled electric drives, high frequency furnaces and high voltage installations. For these and other uses such as, for example, in spaces which involve a danger of explosion, a purely optical distance measuring system and an optical signal transfer associated with the system by means of light conductive fibers appears to be highly advantageous because of the very great frequency separation between the frequencies of the disturbing electrical fields and the optical range of the electromagnetic spectrum, causing the disturbing effect to be practically negligible.
Various fiber optic displacement transducers have been described in the literature including the following articles: N. Lagakos et al, "Applied Optics", Vol. 20, pp. 167-168 (1981); W. B. Spillman and D. H. McMahon, "Appl. Phgys. Lett.", Vol. 37, pp. 145-147 (1980). An optical displacement transducer in the configuration of a Fabry-Perot interferometer appears to be particularly advantageous for digital electronic evaluation, and can be provided with optical fibers for transmitting the signal (Born and Wolf, "Principles of Optics", Pergamon Press, 3d Ed., page 323 et seq. (1965)). A schematic diagram of such a device is shown in FIG. 1 in which the transmission of the light between the two reflectors 4 and 5 as a function of distance "a" between the reflectors is used for the interferometric measurement. Monochromatic light from a light source 1 is supplied through a light conductor 2 and a lens 3 to the reflectors 4, 5. One of the reflectors 4 is fixedly mounted and the other reflector 5 is movable as indicated by the double-headed arrow in such a way that its reflective surface remains parallel with that of reflector 4, the varying distance between and perpendicular to the surfaces 4 and 5 being the component to be measured.
FIG. 2 shows an interferometer arrangement which differs from FIG. 1 in that the input and output light conductors 2 and 7 are on the same side of the reflectors 4, 5 forming the Fabry-Perot interferometer so that only a single lens 3 is needed while, in FIG. 1, an input lens 3 is required and also an output lens 6 is needed. In either case, light is delivered to the interferometer reflectors by fiber 2 and the intensity I(a) of the light which is delivered to output fiber 7 and is received by detector 8 is a periodic function of the distance "a" between the two reflectors 4, 5 of the interferometer, this relationship being graphically illustrated in a simplified manner in FIG. 3a for the interferometer of FIG. 1. For an interferometer according to FIG. 2, the characteristic of the light intensity I(a) has a similar appearance but is characterized by broader maxima and narrower minima. When using a monochromatic light source 1 with the wave length .lambda., the period of change of the intensity I(a) corresponds to a change in the distance "a" of one-half wave length. The light intensity I(a) at the same time varies between a maximal value I.sub.max and a minimal value I.sub.min, and the electrical signal produced by detector 8 varies in proportion to this light intensity. In the subsequent electronic apparatus which process this signal information, a trigger circuit 9 is used which produces a step function or pulse signal in response to the crossing of a predetermined threshold by the input signal. Thus, when the threshold value is established at a mean light intensity value I.sub.s between I.sub.max and I.sub.min, then a pulse signal is produced whenever the intensity value exceeds that threshold. If the distance changes by an amount which corresponds to several half wave lengths, the output of trigger circuit 9 is a corresponding number of impulses, i.e., .DELTA.N=2.DELTA.a/.lambda. pulses. These signals can be counted by an electronic counter 10 and can be recorded in an evaluation unit 11 or can be otherwise evaluated by, for example, a microprocessor involved in a process control. The instrument shown in FIG. 1 thus represents a simple fiber optic incremental distance recorder for the eistance "a" between the two reflectors.
This arrangement, the principal of which is known, has a fundamental disadvantage in that it cannot distinguish between increasing changes of the distance "a" and decreasing changes of that distance. The reason for this lies in the symmetry of the maxima of the light intensity received by the detector 8 and of the detector output signals which are proportional thereto.